1. Field of the Invention
The present invention relates in general to a vane pump, particularly to a variable capacity type vane pump which can vary its pumping capacity by changing the position of a cam ring, and more particularly to a variable capacity type vane pump which is constructed to have inlet and outlet ports at the diametrically opposed portions of the cam ring.
2. Description of the Prior Art
Hitherto, various kinds of variable capacity type vane pumps have been proposed and put into practical use, which are constructed to vary the pumping capacity by changing the position of a part thereof. Some are of a type which comprises an annular rotor driven by an external power source, a plurality of vanes longitudinally movably carried by the rotor, and a cam ring eccentrically disposed about the rotor with its cylindrical inner surface slidably contacting with the leading ends of the vanes. Upon rotation of the rotor and thus the vanes about the axis of the rotor, working fluid is conveyed by each cell defined by the adjacent vanes from an inlet port to an outlet port. During this conveyance, the working fluid is compressed and thus enforcedly discharged to the downstream conduit line of the outlet port. Usually, by the constructional inherency of the pumps of such type, the inlet and outlet ports are formed at the diametrically opposed portions of the cam ring. However, this type pump has the following drawback. That is, under operation of the pump, one half section of the cam ring where the outlet port is formed is biased in one axial direction away from the outlet port by the highly pressurized working fluid in the outlet port, while the other half section of the cam ring where the inlet port is formed is not subjected to such biasing action because the working fluid in the inlet port is relatively low in pressure (that is, about atmospheric pressure). This induces an ill-balanced condition of the cam ring, so that deformation of the cam ring sometimes occurs. With the deformed cam ring, it is impossible to carry out a smooth and accurate pumping operation.